Chain straightening force

Equation (13) is valid for r/Nlp < 0.25 (Fig. 3). At much higher extension ratios, the force must increase indefinitely since the molecule is almost straightened out. The thermodynamic approach to the problem of coil stretching for a freely-jointed chain was considered by Treloar [32], who obtained the following expression for the stress-strain relationship when the two chain ends are kept a distance r apart ... 

Unvulcanized rubber consists of a large number of flexible long molecules with a structure that permits free rotation about single bonds in the primary chain. On deformation the molecules are straightened, with a decrease in entropy. This results in a retractive force on the ends of the polymer molecules. The molecular structure of the flexible rubber molecules makes it relatively easy for them to take up statistieally random conformations under thermal motion. This property is a result of the weak intermolecular attractive forces in elastomers and distinguishes them chemically from other polymers which are more suitable for use as plastics or fibers. 

The swelling of the hydrochloride form is a typical polyelectrolyte behaviour. As the number of ionized sites on the polymer increases, the electrostatic forces tend to straighten the polymer chains and increase the distance between neighboring chains. This movement is greatly restricted in the highly crosslinked ion exchange resin, but does result in the resin particle taking on water. 

Polymer materials in fiber form possess superior mechanical properties compared to the same material in bulk polymer form. The reason for this is clear, the alignment and straightening of the polymer chains make better and better use of the bond stiffnesses and strengths that exist between the individual atoms from which a single polymer molecule is comprised. As the fibers become thinner, this effect is amplified until finally a theoretical maximum is reached. In order to assess the applicability and potential of various SPCs, it is useful to compare both the measured and theoretically achievable (theoretical calculation based on atomic forces) values of tensile strengths and stiffness for various polymer fibers. Table 19.1 presents a selection of thermoplastic polymers, summarizing their mechanical properties in terms of both measured and theoretically achievable fiber properties in comparison with its bulk state [24]. 

Elastomers are amorphous materials. The individual chains of the polymers are random coils that are tangled in an irregular way. The coils straighten out when they are stretched. When the force is released, the elastomer returns to its coiled state because the intermolecular forces are greatest in this arrangement. 

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